This invention relates to improvements in apparatus for depositing a load from a lift truck, of the type equipped with a push frame, so that such load may be deposited in a predetermined position and at a predetermined speed without requiring any substantial level of skill on the part of the lift truck operator.
In the past, it has been particularly difficult for lift truck operators to center a load squarely atop another load of similar size, or otherwise deposit a load precisely in a predetermined position, when utilizing a push frame to push the load forwardly off of the load-carrying forks or platen of the lift truck. The problem arises from the fact that it is most difficult to hold the load stationary above the desired deposit position while pushing it forwardly off of the forks since, in order to do so, the lift truck must be driven in reverse at the same speed at which the push frame is extended relative to the lift truck. Such precise synchronization of two separately controllable variable functions, i.e. speed of extension of the push frame and reverse speed of the lift truck, are beyond the capability of all but the most experienced lift truck operators.
Mercier et al U.S. Pat. No. 2,709,531 suggests one type of solution to this problem. This patent discloses a lift truck push frame extension apparatus which is selectively extensible in response to the reverse travel of the lift truck by selective engagement of a clutch through which the wheels drive the push frame extension apparatus. Because the push frame extension apparatus is driven by the wheels the speed of extension is therefore dependent upon the rearward speed of travel of the truck, the mechanical drive linkage from the wheels being such as to cause the speed of extension of the push frame to equal rearward truck speed. However the foregoing basic principle of operation of the Mercier et al system, i.e. the accomplishment of speed equalization by supplying the power for push frame extension from the lift truck wheels, has a number of drawbacks which render the Mercier principle of operation impractical in modern materials handling applications.
One major drawback is that the push frame cannot alternatively be extended independently of vehicle reverse motion. Since there are many load-handling operations which require such independent extension of the push frame for proper load positioning (for example when handling loads of variable size or when obstacles prevent the lift truck from assuming a position of close proximity to the desired deposit location), such limitation is unacceptable.
In addition a system, such as that shown in the Mercier et al patent, which merely equalizes the speed of extension of the push frame with the reverse speed of the lift truck does not control the magnitude of the speed at which the load is deposited. This is still left to the operator whose lack of experience can permit him, for example, to perform the load deposit operation too rapidly for a fragile type or high weight of load which might be involved, and thereby possibly cause load damage, mispositioning of the load or a disturbance of the stability of the load stacks.
Furthermore such a system requires a clutch-engagement step prior to actuating the depositing function. Such extra step consumes time which, under modern conditions of high-volume materials handling, is multiplied many times and decreases the overall efficiency of the handling operation. Moreover, any extra preparatory step is susceptible to being inadvertently omitted prior to moving the lift truck rearwardly, in which case the lift truck would have to be repositioned and the procedure repeated.
Finally, a great many lift truck push frames are now equipped with slip sheet clamps, the clamping and release of which are interrelated with the extension and retraction apparatus of the push frame in such a way as to cause variable time delays between actuation of the extension apparatus and actual extension of the push frame, thereby making it much more difficult to achieve speed and movement synchronization of the push frame and lift truck unless the power sources for the two functions are separate from one another.
It should be noted that different types of agricultural equipment, such as those shown for example in De Wall U.S. Pat. No. 2,531,560 or Mader et al U.S. Pat. No. 3,110,148 have been devised in the past which also have load-pushing mechanisms powered by, and therefore synchronized with, the wheels of the vehicle. However these essentially operate on the same principle as that of the Mercier et al patent, and therefore do not suggest solutions to the foregoing problems.
What is needed therefore is an improved lift truck load-depositing apparatus which not only equalizes and synchronizes the speed of push frame extension with the reverse speed of the lift truck to obtain precise positioning of a deposited load, but which in addition provides separately operable power sources for lift truck movement and for extension of the push frame respectively, provides not only equal truck and push frame speeds but also a predetermined fixed magnitude of such speeds, operates automatically in response to actuation of push frame extension rather than in response to lift truck rearward motion and provides the necessary synchronization of lift truck and push frame movement despite time delays inherent in the extension of the push frame due to the presence of a slip sheet clamp release function.